The same has happened to me many times. Sometimes the part is found - sometimes not.
About twenty odd years ago, I dropped the sear spring screw from a Siler lock, after polishing, hardening and tempering it, and it disappeared into the void. Last May, I renovated my shop, and one morning, I looked down at the floor at my feet, and there it was. The floor has been swept and vacuumed dozens of times since the loss, but somehow the spring ended up in a safe little crack or cranny until then.
The fly from a lock tumbler is one of the worst items to misplace.